The present invention relates to random access multipoint data communication systems, and more specifically to a slotted ALOHA system in which packets from remote stations are randomly transmitted over predefined time slots.
The unslotted ALOHA system is the first random access multipoint satellite data communication system. This system uses a single radio channel which is shared by a plurality of remote stations. Whenever a remote station generates a packet, it transmits the packet on the common radio channel to a central station. Since more than one station may attempt to transmit a packet simultaneously, several transmissions may overlap. These overlapping transmissions are said to collide if any portion of two packets overlap. Whenever a collision occurs, random numbers are used to specify a period of time each conflicting station must wait before an attempt is made to gain access to the channel. To increase channel utilization, the slotted ALOHA system was proposed in which the channel is partitioned into equal time slots and each station only transmits a packet at the beginning of a slot. In this way overlapping transmissions are forced to completely overlap. This technique substantially doubles the maximum channel utilization of the unslotted ALOHA system.
Since the remote stations transmit their packets only if there is a need to transmit, the system has no way of knowing if there is a remote station incapable of transmitting no packets due to a trouble.
It is thus desirable that the data communication system of this type be capable of detecting which one of the remote stations is not functioning properly to allow necessary action to be taken as earl as possible.